package lanqiao;

import java.util.Scanner;

public class 饮料换购 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        System.out.println(process(n));
    }
    private static int process(int n){
        if (n<3){
            return n;
        }
        return 3+process(n-2);
    }
}
